This invention relates to the preparation of pure chemical gypsums which are produced in the manufacture of phosphoric acid by wet processes. The gypsum can be refined in a process separate from the phosphoric acid production or during the phosphoric acid process itself. The process of the invention leads directly to pure calcium sulfate semihydrate.
Phosphoric acid processes yield chemical gypsums which have a more uniform composition than natural gypsums but which are unusable for many purposes as a result of impurities, especially phosphoric acid in insoluble and water-soluble form. This is especially true with regard to the use of such gypsums in the construction and cement industry.
The P.sub.2 O.sub.5 impurities in commonly used phosphoric acid processes amount to from one to two percent of the weight of the dry calcium sulfate. 80% of the P.sub.2 O.sub.5 impurities are usually in insoluble form, and about 20% in soluble form. The soluble phosphoric acid can be removed by further washing with water, but not the insoluble.
The state of the art furthermore includes phosphoric acid processes in which pure gypsums are produced in the form of calcium sulfate dihydrate crystals.
In such a process, for example, first semihydrate is precipitated and then a recrystallization to large dihydrate crystals is performed, the latter being then removed by filtration. In this procedure it is possible to reduce the content of phosphoric acid in the end product to about 0.4%. This phosphoric acid content, however, is still too high for the cement industry.
The semihydrate can be obtained directly in phosphoric acid processes of this kind through the use of mixtures of phosphoric and sulfuric acid. In such processes, however, a rather high phosphoric acid content in the gypsum must accordingly be expected.
The stability range of semihydrate in relation to nitric acid concentration and temperature was determined by experiment, the products formed under the given reaction conditions (nitric acid concentration and reaction temperature) being tested to determine whether they were still capable of setting upon the addition of water. In the experiments, allowance was made, in stating the nitric acid concentration, for the diluting effect produced by the water present in the dihydrate.
In representing the results of the experiments it was found to be desirable not only to plot the concentration c against the minimum temperature t at which the transformation takes place (FIG. 1), but also to plot this temperature against the function 100/c. As the corresponding FIG. 2 shows, the points of measurement arrange themselves virtually on a straight line, and in the range of between 20 and 100.degree. C. and in the concentration range above 30 wt-% HNO.sub.3 (100/c&lt;3.333), the equation EQU 100/c = mt + b (1)
applies with good approximation, the constants m and b having the following values: EQU m [(= tan .alpha.)] = 0.02853 (2) EQU b = 0.7004 (3)
The stability range of calcium sulfate semihydrate thus lies below this straight line, and that of the dihydrate lies above it.